Loading telephone conductor



July 23, 1929. R. GOLDSGHMIDT LOADING TELEPHONE CONDUCTOR Filed April 2:, 192a ATTOYRNEYI Patented July 23, i929.

GOLDSCHMIDT ,9 Blllll'aIN-Kiifilfil-IOBST, GERMANY,'ASSIG1\TOB "1'0 GENERAL ELECTRIC COMPANY, A CGBPOBAJIQLI NEW YURK.

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Application filed. April 23, 1928, Serial No. 272,159, in Germany May ll, 1927.

This invention relates to the loading of telephone circuits, and the like, and deals particularly with a method of equalizing induct-ive unbalance due to such loading.

In loading telephone conductors, one of the main requirements is that the loading coils used in the several conductors be, as far as possible, symmetrical in order to avoid disturbing cross-talk. It is desirable not only to have the resistances of the capacities in balance, but also to have symmetry with respect to the inductance in the opposite sides of the circuits. v

The invention will be explained in connection 'ith accompanying drawings in which. Figure 1 illustrates diagrannnatically an ordinary loading coil with windings in the opposite sides Of a. telephone circuit, and Fig. 2 represents two such coils in a telephone circuit intended to be used in phantom connections, wherein one oi the coils is eiiective in the side circuit, and the other is effective in the phantom circuit.

In the manufacture of loading coils, the greatest care is taken to make the coils alike, both with respect to each other and with respect to the several windings on a coil, so as to maintain the inductive as well as the other balances essential to good practice. As is well known, however, it is impossible to commercially produce coils in which perfect symmetry is attained.

Referring to Fig. l, the inductive effect of winding L, may be represented by the eX- pression L i rl and the inductance of winding L is L ild where L is the self-inductance and M the mutual inductance l etween the two coils. The dissymmetry due to the unavoidable divergence between the inductance of the two coils may therefore be represented by the expression AL L L Now, with a given dissymn'ietry between the two windings, the smallest change which can be made in the inductance of either winding is by the removal or addition of a single turn to that winding, and this is, in general, too coarse an adj ustmentto produce the symmetry necessary for high-grade transmission. In working with phantom circuits, two coils are employed, as indicated in Fig. 2, per pair of conductors, of which the coil L, L, may be etlective in the side circuit, and the coil L L may be in the phantom circuit. Each of these coils will have a dissymmetry,

and it has heretofore been the practice to improve the symmetry by removing or adding one or more turns to one of the coils. It is also possible to compensate the dissymmetry to a certain extent by the select-ion of two windings for connection in a given wire, having as nearly opposite dissymmetries as are found a"\"-'ailable. This, however, renders necessary a tedious selection of the necessary windings, and the result depends upon the possibility of finding windings suited to each other. Its possibility becomes less the smallthe total number of windings there are available. A further means for compensating asymmetries is in the utilization of the stray fields. This means fails, however, when the stray fields are slight.

The underlying idea of the present invention is to accomplish a finer adjustment of the inductance than is possible by the removal or addition of a single winding by using the difference in inductive effect between the winding of one coil and the winding of an associated coil in a loading unit. Where we have two associated coils, as in Fig. 2, one of which is effective in the side circuit, and one in the phantom circuit, it is well known that the inductances of the respective coils are in general different, and the crosstalk from side to phantom circuit, due to a single unbalanced turn in the two coils, is different. For example, in a known type of loading coil, the crosstalk due to an unbalanced turn in the phantom coil is 216 parts in a million, while that due to an unbalanced turn in the associated side circuit coil is but 172'parts in a million. The smallest adjustment which can be made,- therefore, by the removal or addition of a single turn to either of these coils may be represented by these figures.

According to the present invention, it is proposed to obtain a finer adjustment by adding a turn to one of these coils while removing a turn from the associated coil of the same loading unit, thereby getting a. total adjustment equal to the difference between the inductive effect of the two coils. In the example above given, for instance, the total change in inductance would be represented by it unbalance units, which, it will be seen, is much smaller than the adjustment obtainable by the removal or addition of a single turn to either the phantom or side circuit coil. The process is obviously applicable to loading units Whether the phantom loading is accomplished by tWo associated phantom coils or by one four Winding phantom coil.

It is obvious that an adjustment greater than that represented by the difference in in ductive effect of the tWo windings, as above illustrated, may be obtained by adding and subtracting tWo, three or more turns to the respective associated coils instead of a single turn.

What I claim is:

1. The process of producing inductive symmetry in loading units having a plurality of coils in each line conductor, which consists in adding one turn to one of the coils, and,' at the same time, removing aturn from another of the associated coils in a given conductor.

2. The process of producing inductive symmetry between loading units having a plurality of coils in each line conductor, Which consists in adding to one of the coils a certain number of turns and removing from another of the associated coils in the same line conductor, a like number of turns.

In Witness whereof, I have hereunto set my hand this 31st day of March, 1928.

ROBERT GOLDSGHMIDT. T 

